ACTION, OF THE SILVER GRAIN. 41 



so delicate can, in general, be removed from their 

 natural situation, without the destruction of that fine 

 irritability on which such a motion must depend. \Te 

 may also take into consideration the agitation of the 

 vegetable body by winds, which is known by expe- 

 rience to be so wholesome to it*, and must serve 

 powerfully to propel the fluids of lofty trees ; the pas- 

 sage, and evolution perhaps, of air in other parts or 

 vessels, surrounding and compressing these; and lastly 

 the action, so ingeniously supposed by Mr. Knight, of 

 those thin shining plates called the silver grain, visible 

 in oak wood, which pressing upon the sap-vessels, and 

 being apparently susceptible of qufck changes from 

 variations in heat or other causes, may have a power- 

 ful effect. " Their restless temper." says Mr. Knight, 

 " after the tree has ceased to live, inclines me to be- 

 lieve that they are not made to be idle whilst it con- 

 tinues alive." Phil. Trans, for 1801, p. 344. These 

 plates are presumed by the author just quoted to be 

 peculiarly useful in assisting the ascent of the sap 

 through the alburnum of the trunk or chief branches, 

 where indeed the spiral coats of the vessels are either 

 wanting, or less elastic than in the leaf-stalks and 

 summits of the more tender shoots. 



However its conveyance may be accomplished, it is 

 certain that the sap does reach the parts above men- 

 tioned, and there can surely be now as little doubt of 



* See Mr. Knight's experiments in confirmation of this in the Phil. 

 Trans, for 1803, p. 280. 



